1. Field of the Invention
The present invention relates to a transflective liquid crystal display apparatus and, more specifically, to a transflective liquid crystal display apparatus in which each pixel comprises a light-reflection type reflective area and a light-transmission type transmissive area.
2. Description of the Related Art
FIG. 10 illustrates a conventional case depicted in Japanese Unexamined Patent Publication 2003-344837 (Patent Literature 1). A liquid crystal display apparatus 100 shown in FIG. 10 is constituted with a lower side substrate 11, a counter side substrate 12, and a liquid crystal layer 13 held by being interposed therebetween. Further, each of the pixels constituting the display unit of the liquid crystal display apparatus 100 comprises a light-reflection type reflective area and a light-transmission type transmissive area. FIG. 10 shows a schematic sectional view of a single pixel that is disclosed in Patent Literature 1.
In FIG. 10, the counter side substrate 12 is constituted with a black matrix layer 17 formed on a transparent insulating substrate 22b as a light-shielding film, a color layer 18 that is partially overlapped with the black matrix layer 17, and a transparent overcoat layer 19 formed on the black matrix layer 17 and the color layer 18. Further, in order to prevent the liquid crystal layer 13 from being electrically influenced by the electrification from the surface of a liquid crystal display panel generated due to a contact or the like, a transparent conductive layer 15 is formed on the back face of the transparent insulating substrate 22b. The color layer 18 is formed with a resin film containing dyes or pigments of red (R), green (G), and blue (B).
Further, the lower side substrate 11 comprises: on a transparent insulating substrate 22a, a first metal layer where scanning lines (not shown) and gate electrodes (not shown) of thin film transistors used for driving are formed; a first interlayer insulating film 23 formed thereon; a second metal layer formed on the first interlayer insulating film 23, on which data lines 24, source electrodes and drain electrodes (not shown) of the thin film transistors are formed; a second interlayer insulating film 25 formed thereon; and common electrodes 26 and pixel electrodes 27 formed thereon with transparent electrodes.
The lower side substrate 11 and the counter side substrate 12 respectively comprise an alignment film 20a and an alignment film 20b on the respective opposing face sides thereof. Rubbing processing is applied thereon from the extending direction of the pixel electrode 27 and the common electrode 26 towards a prescribed direction tilted by about 10 to 30 degrees so that the liquid crystal layer 13 is aligned homogeneously. Thereafter, both substrates are laminated to face each other. This angle is called an initial alignment direction of the liquid crystal molecules.
A spacer (not shown) is provided between the lower side substrate 11 and the counter side substrate 12 for keeping the thickness of the liquid crystal layer 13. Further, a seal (not shown) is formed in the periphery of the liquid crystal layer 13 for not leaking the liquid crystal molecules to the outside.
In addition to the data lines 24 through which data signals are supplied, common electrode wirings (not shown) and the common electrodes 26 through which reference potential is supplied, and the pixel electrodes 27 that correspond to the pixels to be displayed, the lower side substrate 11 comprises scanning lines (not shown) through which scanning signals are supplied and the above-mentioned driving thin film transistors (TFTs) (not shown) which are provided on the transparent insulating substrate 22a. 
A driving thin film transistor comprises a gate electrode, a drain electrode, and a source electrode, and it is provided by corresponding to each pixel in the vicinity of the intersection between the scanning line and the data line 24. The gate electrode is electrically connected to the scanning line, the drain electrode to the data line 24, and the source electrode to the pixel electrode 27.
The common electrode 26 and the pixel electrode 27 are both in a pectinate shape, and the teeth of each electrode are all extended in parallel to the data line 24. Furthermore, the teeth of the common electrode 26 and that of the pixel electrode 27 are arranged alternately with each other.
An in-plane switching system is employed for both of the above-mentioned transmissive area T and reflective area H of the liquid crystal display apparatus 100. Regarding the liquid crystal display apparatus 100, in the pixel to which the data signals (selected by the scanning signals supplied through the scanning lines, and supplied through the data lines 24) are written, parallel electric fields are generated in the above-described transparent insulating substrates 22a, 22b between the common electrodes 26 and the pixel electrodes 27. The alignment direction of the liquid crystal molecules is rotated within a plane in parallel to the transparent insulating substrates 22a, 22b in accordance with the generated electric field so as to perform a prescribed display.
A vertically long area surrounded by the above-described common electrode 26 and the pixel electrode 27 is called a column (not shown). In the above-described liquid crystal display apparatus 100, the common electrode 26 and the pixel electrode 27 are both formed with a transparent material, indium tin oxide (ITO).
Further, in the transmissive area T and the reflective area H, the common electrode 26 is formed on a layer that is closer to the liquid crystal layer than to the scanning line and the data line 24, and it is formed to have a wider width than the scanning line and the data line 24 so as to cover the scanning line and the data line 24 completely.
Furthermore, as shown in FIG. 10, in the reflective area H, a reflective plate 9 is formed on a layer that is closer to the liquid crystal layer than to the scanning line and the data line 24, and it is disposed to cover the scanning line and the data line 24 completely.
By forming the common electrode 26 and the reflective plate 9 in this manner, it is possible to shut the leaked electric field from the data line 24 and the scanning line. Thus, the effective display area that can be controlled by the electric field generated between the pixel electrode 27 and the common electrode 26 can be expanded. Therefore, the aperture ratio can be improved.
Furthermore, as can be seen from FIG. 10, the second interlayer insulating film 25 is provided between the common electrode 26 and the data line 24 in the transmissive area T.
Through setting the ratio d/ε of the film thickness (d) with respect to the permittivity (ε) of the second interlayer insulating film 25 sufficiently large, the parasitic capacitance between the data line 24 and the common electrode 26 can be decreased. Further, as can be seen clearly from FIG. 10, in the reflective area H, the second interlayer insulating film 25, a second insulating film 8b, the reflective plate 9, and a third insulating film 8c are provided between the common electrode 26 and the data line 24. This provides a proper distance between the data line 24 and the common electrode 26, thereby decreasing the parasitic capacitance.
In this conventional case, the common electrode 26 and the pixel electrode 27 are both formed on the second interlayer insulating film 25 in the transmissive area T, and the common electrode 26 and the pixel electrode 27 are both formed on the third insulating film 8c in the reflective area H. Therefore, the common electrode 26 and the pixel electrode 27 can be formed by the same step and the same material, which improves the manufacture efficiency.
Further, after forming the interlayer insulating film 25, a second insulating film 8b is formed in the reflective area H. The second insulating film 8b is normally formed with a double-layer structure constituted with an uneven film and a flattening layer. However, it can also be formed with a single-layer structure by using a halftone mask.
Furthermore, the reflective plate 9 made of aluminum is formed on the second insulating film 8b whose surface is uneven. This reflective plate 9 functions to reflect the incident light diffusely. The third insulating film 8c is formed over the reflective plate 9 and the surface thereof is flattened. Further, the common electrode 26 and the pixel electrode 27 made of indium tin oxide (ITO) as in the case of the transmissive area T are formed on the third insulating film 8c, and an alignment film 20a is formed thereon to constitute the lower side substrate 11.
In the above-described conventional case shown in FIG. 10, a thin flattening film is provided on the uneven reflective plate 9 in the reflective area H, and interdigital electrodes are provided thereon. Meanwhile, the transmissive area T has a structure where the interdigital electrodes of the same layer as that of the reflective interdigital electrodes are directly formed on the second interlayer film (without providing the uneven layer and the flattening layer).
There is a difference in heights provided between the transmissive part and the reflective part (reflective part-transmissive part difference) by the difference of “uneven layer+reflective part metal+flattening layer” to provide a prescribed retardation (phase difference between two kinds of intrinsic polarization light) between the reflective part (Δnd (R)) and the transmissive part (Δnd (T)) by that difference.
The refractive index anisotropy of the liquid crystal (Δn) is about Δn=0.1. Thus, provided that Δnd (T)−Δnd(R)=(λ/2)−(λ/4)=137 nm, it is necessary to provide the reflective part-transmissive part difference of about 1.3 μm. In this case, about 0.1-0.3 μm is required for the film thickness of the reflective part metal (aluminum), so that the thickness of the “uneven layer+flattening layer” becomes about 1.0 μm.
Further, in the above-described conventional case, there is no restriction set in the reflection mode regarding the angle of the incident light and the angle of the exit light with respect to the liquid crystal.
The angle of the incident light and the angle of the emitted light with respect to the liquid crystal at the time of the reflection mode in the above-described conventional case will be investigated herein.
In the above-described conventional case, the in-plane switching system is employed for driving the liquid crystal. In this drive method, it is necessary to set the pretilt angle of the liquid crystal to be close to 5 degrees or less (preferably 0 degree) as much as possible.
This is because of the following reasons. That is, if the pretilt of the liquid crystal is too large, idealistic in-plane switching drive cannot be performed and there generates alignment disturbances, since the liquid crystal keeps the pretilt to be aligned in a tilted manner on the substrate surface. Therefore, contrast and the viewing angle are deteriorated, thereby deteriorating the display quality.
Now, the relation regarding the settings of the above-described tilt angle of the unevenness, the incident angle, the exit angle, and the operation of the entire apparatus will be analyzed further.
When the incident angle of the light is as shallow as 0-15 degrees and the exit angle is 0 degree, the tilt angle of the uneven reflective plate can be set as shallow, and the film thickness of the unevenness may be set as about 0.5 μm. For easing the difference in the heights of the uneven parts, it is necessary to provide a flattening film having the thickness about the same as that of the uneven film. Thus, the thickness of the flattening film also needs to be about 0.5 μm.
When the tilt angle of the uneven reflective plate is gentle as in this case, the rise and fall of the uneven parts on the surface of the flattening film under the interdigital electrodes is relatively gentle even if the uneven film or the flattening film is thin, and the pattern formation of the interdigital electrodes is relatively easy. Further, there causes no disturbance even when the liquid crystal is driven in the lateral electric field, and there is not much deterioration generated in the display quality mentioned above.
In the meantime, when the incident angle of the light is as deep as 30-15 degrees and the exit angle is 0 degree, and the tilt angle of the uneven reflective plate is deep, it is necessary to thicken the uneven film. In this case, the difference between the uneven parts on the surface of the flattening film under the interdigital electrodes becomes also significant depending on the material and the thickness of the flattening film. Therefore, the pattern formation of the interdigital electrodes becomes difficult. Further, there often causes disturbance in the liquid crystal when the liquid crystal is driven in the lateral electric field, which often results in causing deterioration of the contrast and the viewing angle. Therefore, it is necessary in such a case to form the flattening film to be thick.
As described above, when a priority is given to the flattening characteristic under the interdigital electrodes in the reflective area, the uneven film and the flattening film both become thick. Thus, in the structure of the above-described conventional case, it is difficult to set the reflective part-transmissive part difference to be about 1.0 μm, for example. Therefore, in the conventional transflective liquid crystal display apparatus of an In-plane switching mode, it is difficult to obtain the reflective characteristic of wide view angles.